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The other day in class we were discussing the enantiomers of 1,2-dichlorocyclohexane and my teacher told us that (R,R) and (S,S) are different isomers while (R,S) and (S,R) are the same.

I understand how (R,S) and (S,R) are the same (Because they can be rotated) but I don't understand why (S,S) and (R,R) are not the same, even though they are the same if you rotate them too.

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  • $\begingroup$ Classical example - you can also rotate your hands, but that wouldn't make them identical. $\endgroup$ Commented Oct 9, 2017 at 8:01
  • $\begingroup$ The point is that you cannot rotate them. Rotation doesn't change the identify of the stereocenter. That's why rotation of (R,S) becomes (S,R). But rotation of (S,S) is still (S,S). $\endgroup$
    – Zhe
    Commented Oct 16, 2017 at 23:58

1 Answer 1

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Left: RR Right:SS

The one on the left (RR) and the right (SS) are not superimposable mirror images of one another and are thus enantiomers. In your head, try flipping one of them. For instance, if you flip the RR molecule on the left as I drew it, it will look something like this: Left:RR Right:SS

The two pictures I posted are depicting two same molecules, except the one on the left was flipped around an axis in plane of the screen. With the second picture, it's easier to see that these two molecules are non-superimposable mirror images. Also notice how neither of these molecules have a plane (or point) of symmetry.

I recommend trying to build this with a model kit if you can't visualize it in your head easily.

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  • $\begingroup$ I actually want to know how two o,p dichloro cyclohexane with two chlorides projecting out are different from two projecting inwards. I can visualise this thanks $\endgroup$
    – Beast
    Commented Oct 9, 2017 at 12:48
  • $\begingroup$ RS and SR are the same molecules. There's a plane of symmetry going through the molecule. Since they also have chiral centers, you could characterize them as meso. $\endgroup$
    – orchid
    Commented Oct 9, 2017 at 14:54
  • $\begingroup$ The same goes for the two molecules given in the answer you could view one from under the plane of the screen and they're the same $\endgroup$
    – Beast
    Commented Oct 9, 2017 at 14:57
  • $\begingroup$ No, that is false. There is no plane of symmetry for the molecules I drew pictures of, and their mirror images aren't the same. If you view the left molecule on the top picture from the other side of the plane of the screen, you'll see the left molecule in the lower picture. That is not the same molecule as the molecule on the right. I think you're having trouble visualizing the molecules in 3D. $\endgroup$
    – orchid
    Commented Oct 9, 2017 at 15:35
  • $\begingroup$ Yes, you're right. Sorry, my bad. I didn't visualise it properly $\endgroup$
    – Beast
    Commented Oct 9, 2017 at 16:05

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